NEW YORK -- Forced to confront an unexpectedly stiff challenge from former California Gov. Jerry Brown Jr., Arkansas Gov. Bill Clinton is trying to keep his foe at bay in the critical New York primary campaign by slamming his flat tax proposal and sparring with him in a series of debates.

The two-pronged effort is partially aimed at diverting attention from Mr. Clinton's own Achilles' heel in the battle for the Democratic presidential campaign: the character issue.

A statewide poll released yesterday suggests that the strategy is paying off. The survey, conducted by the Marist College Institute of Public Opinion Sunday and Monday, put Mr. Clinton ahead of Mr. Brown, 36.7 percent to 26 percent, among likely voters in the Democratic primary. The poll's margin of error is plus-or-minus 5 percentage points.

Although the results were good news for the beleaguered Clinton campaign, the lead represents a decrease of about 6 percentage points from his advantage over Mr. Brown in a poll taken March 24.

And most political analysts agree that the outcome of Tuesday's New York vote remains in doubt as Mr. Brown presses his anti-establishment appeal to discontented voters. The new poll showed 25 percent of the likely Democratic voters are undecided, giving Mr. Brown a large audience that might be receptive to his message.

But, so far, the Clinton campaign again appears to be demonstrating the flexibility and resiliency that saved his candidacy from extinction in the Feb. 18 New Hampshire primary after he faced unsubstantiated allegations of adultery and questions about his draft status during the Vietnam War.

Mr. Clinton's more recent political troubles were sparked by Mr. Brown's upset victory in last week's Connecticut primary. Mr. Brown came roaring into New York on a wave of momentum. Mr. Clinton, meanwhile, found himself initially on the defensive, forced to repeatedly apologize for having played golf two weeks ago at a segregated country club.

Referring to the centerpiece of Mr. Brown's domestic program -- his call for a single 13 percent income tax rate and a 13 percent value-added tax, Clinton New York campaign manager Harold Ickes said, "We have to go after Brown on his flat tax idea."

Mr. Clinton's second imperative, Mr. Ickes said, was to keep attention on issues rather than character questions. "Bill Clinton has to lay out his program for running the country as president," Mr. Ickes said.

The new Marist Institute poll indicates that the focus on Mr. Brown has had an effect. The survey found that the percentage of likely Democratic voters with an unfavorable impression of Mr. Brown increased 10 percentage points -- to 32.3 percent -- from in the March 24 survey. Mr. Brown's favorable ratings, meanwhile, dropped 10 percentage points to 49.8 percent.

Lee Miringoff, the institute's director, attributed the shift to the flat tax issue. "That has been the focus of Clinton's critique of Brown and to the extent that it has been heard, it has hurt,"he said.

The poll also found that Mr. Clinton's unfavorable rating had remained at about 31 percent, despite his disclosure that he had tried marijuana while attending college in England.

Interviews with voters attending a Brown rally at Union Square in lower Manhattan yesterday tended to bear out Mr. Miringoff's judgment on the impact of the flat tax issue.

"It's hard to get details of how that thing works," said Joey Davis, a computer programmer who said he might support Mr. Brown but was troubled about the implications of the flat tax proposal.

Even as they put the spotlight on the flat tax plan, Mr. Clinton's advisers became alarmed by the stream of nasty headlines in New York City tabloids, especially after his belated admission that he had smoked marijuana. So they decided to modify the second part of their strategy. Not only would Mr. Clinton stress his ideas for running the country, but he would do so in a series of debates with Mr. Brown.

Judging from the response to the first debates, which began Tuesday, these sessions seem likely to dominate press attention. Thus, they could drive the issue of Mr. Clinton's character into the background.